1. Field of the Invention
The present invention relates to an image transferring device for an image forming apparatus such as a copier, printer, facsimile transceiver or similar photographic image forming apparatus in which an image is electrostatically formed on an image carrier. More particularly, the invention provides an image transferring device and a method for manufacturing an image transferring device in which an image is transferred from an image carrier to a transfer sheet while the transfer sheet is electrostatically adhered to and transported by a transfer belt.
2. Description of the Related Art
Japanese Patent Laid-Open Publication No. 6-3971 discloses a conventional image transferring device for an image forming apparatus, with the transfer device provided for an image forming apparatus such as a copier, or a printer. Referring to FIG. 4, with such an image forming apparatus, a transfer belt 2 is disposed below a photosensitive drum 1 and passes over a conductive drive roller 5 and a conductive driven roller 4. The conductive drive roller 5 is connected to a motor, not shown, and is rotated in a direction indicated by an arrow in the figure. As the conductive drive roller 5 is rotated, the transfer belt 2 is moved in a direction for transferring a transfer sheet (indicated by the arrow in the figure).
A bias roller 3 is located downstream of the conductive driven roller 4 with respect to the moving direction of the transfer belt 2. The bias roller 3 is held in contact with an inner surface of the transfer belt 2. In addition, a ground plate 6 is located upstream of the conductive driven roller 4 with respect to the moving direction of the transfer belt 2, and is connected to ground so as to allow a flow of electric current from the transfer belt 2 to ground. A power source 7 is connected to the bias roller 3, and applies a charge/current to the transfer belt 2 which is opposite in polarity to that of the toner deposited on the photosensitive drum 1. A resistor 9 is provided between the ground plate 6 and ground. A current control unit 10 is provided so as to control the output of the power source 7. The electric current is fed to the transfer belt 2 via the bias roller 3 from the power source 7. In addition, an eraser (not shown) is disposed near the conductive driven roller 4 so as to remove the charge from the transfer belt 2 by irradiation.
In operation, the transfer sheet is delivered from a paper feeding device, not shown. The transfer sheet is polarized/charged by the charge applied from the bias roller 3 via the transfer belt 2. The transfer sheet is thus adhered onto the transfer belt 2 by the electrostatic charge. A toner image is transferred from the photosensitive drum 1 to the transfer sheet and the transfer sheet on which the toner image is formed is delivered by the transfer belt 2. The transfer sheet is then separated from the transfer belt 2 at the location of the conductive drive roller 5 by the rigidity of the transfer sheet. This separation is also known as a curvature separation (i.e., the sheet separates as it passes over the curvature of the roller).
Japanese Patent Laid-Open Publication No. 6-3972 discloses another conventional image transferring device for an image forming apparatus, with the transfer belt device provided for an image forming apparatus such as a copier, or a printer. As shown in FIG. 5, this arrangement includes a current control unit 11, provided to control the output of the power source 7. The remaining elements are designated with reference numerals as discussed with reference to FIG. 4, and therefore a description of these elements is omitted.
With such a current control unit, the current flowing from the transfer belt to the drum can be maintained constant. For example, assume that an output current flowing from the power source to the transfer belt 2 via the bias roller 3 is I-1, and that a feedback current flowing from the transfer belt 2 to the current control unit 11 via the ground plate 6 is I-2. The output current from the power source is controlled so as to satisfy a following equation: EQU I-1-I-2=K
where K is constant.
With this relationship, current flowing from the transfer belt 2 to the photosensitive drum 1 remains constant and the toner image can more reliably be transferred to the transfer sheet under a stable transfer condition.
However, in the foregoing arrangements, the transfer belt 2 is repeatedly bent, and also expands after long periods of use. As a result, the structural state of the transfer belt 2 becomes unstable. For example, a coated portion on an outer surface of the transfer belt 2 can partially peel off, or a crack can occur on the outer surface of the transfer belt 2 due to deterioration over time. With such deterioration, a portion (e.g., the peeled-off or cracked portion) has a low-resistance value as compared with surrounding peripheral portions. In this condition, with the surface of the transfer belt 2 in pressure contact with the photosensitive drum 1, a large current flows at the deteriorated portion of the transfer belt 2, and abnormal leakage occurs between the transfer belt 2 and the photosensitive drum 1. Such abnormal current leakage can cause pin-hole damage to the photosensitive drum 1, and/or blanking of an image (i.e., a portion of an image is not formed) due to damage of the drum. Further, deterioration or damage to the belt or drum can cause the transfer rate to decrease.